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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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Thermocouples

Thermocouples work on the Seebeck principle, discovered accidentally in 1822 by an Estonian physician named Thomas Seebeck. Basically, the Seebeck principle explains how two dissimilar metals generate a temperature-dependent flow of current at their welded junction when attached to a closed circuit.

Seebeck discovered that a compass needle would be deflected when a closed loop was formed of two metals joined in two places with a temperature difference between the junctions. The metals respond to temperature by assuming different voltage potentials, thereby generating a current related to the temperature of the junction.

This table lists the properties of the most common thermocouple junctions.

Type	Metals	ASTM Color	EMF (mV) Over Temp Range	Temperature Range
B	Platinum30% Rhodium (+) Platinum 6% Rhodium (-)	Gray Red	0 to 13.820	0 to 1700°C (32 to 3092°F)
C	W5Re Tungsten 5% Rhenium (+) W26Re Tungsten 26% Rhenium (-)	White Red	0 to 37.066	0 to 2320°C (32 to 4208°F)
E	Chromel (+) Constantan (-)	Violet Red	-9.835 to 76.373	-200 to 900°C (-328 to 1652°F)
G	Tungsten 26% (+) Rhenium W-26% (-)	White Red	0 to 38.564	0 to 2320°C (32 to 4208°F)
J	Iron (+) Constantan (-)	White Red	-8.095 to 69.553	0 to 750°C (32 to 1382°F)
K	Chromel (+) Alumel (-)	Yellow Red	-6.458 to 54.886	-200 to 1250°C (-328 to 2282°F)
N	Nicrosil (+) Nisil (-)	Orange Red	-4.345 to 47.513	270 to 1300°C (-450 to 2372°F)
R	Platinum 13% Rhodium (+) Platinum (-)	Black Red	-0.226 to 21.101	0 to 1450°C (32 to 2642°F)
S	Platinum 10% Rhodium (+) Platinum (-)	Black Red	-0.236 to 18.693	0 to 1450°C (32 to 2642°F)
T	Copper (+) Constantan (-)	Blue Red	-6.528 to 20.872	-200 to 350°C (-328 to 662°F)

Thermocouple types R, S, and B are constructed of platinum and rhodium, and are referred to as noble metal thermocouples. They are more accurate and more stable than base metal types, but are more expensive.

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